Pulmonary embolism, in which emboli from any of various regions of the vascular system pass into the lungs, accounts for thousands of deaths each year in the United States. Blood clots from the lower extremities are commonly carried to the heart through the inferior vena cava, and thence to the lungs.
Many patients with documented pulmonary embolism can be treated with anti-coagulants to prevent further formation of thrombi, but there are situations in which mechanical interruption of the inferior vena cava is the preferred method to prevent pulmonary embolism. To prevent blood clots from passing upwardly through the inferior vena cava, it has been suggested to place filters in the vena cava which filter out blood clots. An excellent but early description of vena cava filters is found in Palestrant, Aubrey, M, et al. Comparative In Vitro Evaluation of the Nitinol Inferior Vena Cava Filter, Radiology 145:351-355, November, 1982. A more recent treatment of the subject of vena cava filters is found in Interventional Radiology, 3.sup.rd Edition, Castaeda-Zuniga, Wilfredo R., ed., Williams & Wilkins, Baltimore, pp 854-896 (1997). Various filters are disclosed in Lebigot, U.S. Pat. No. 4,781,177, Simon, et al., U.S. Pat. No. 5,669,933 and Maderlinger, U.S. Pat. No. 5,720,764. Reference may be made also to Rasmussen, et al., U.S. Pat. No. 5,133,733, LeFebvre, U.S. Pat. No. 5,108,418, Goldberg, et al. U.S. Pat. No. 5,152,777 and El-Nounou, et al. U.S. Pat. No. 5,242,462.
In general, vena cava filters are introduced into the vasculature through a puncture or an incision in a major vessel such as the internal jugular vein and the filter, elastically restrained in a delivery catheter, is passed from the jugular vein through the right atrium of the heart and into the inferior vena cava whereupon the filter is mechanically expelled from the catheter and expands into contact with the lumen. Various hook-like projections have been suggested for use in holding the filter in place once the delivery catheter has been withdrawn. When a filter using hooks, barbs and the like to retain it in place permanently is thus to be removed, surgical intervention is usually required.
Temporary introduction of a vena cava filter may be desired to provide rapid protection against pulmonary embolism, but as the condition producing blood clots is successfully treated, it may be desired to remove the filter from the vena cava. Vena cava filters commonly include a hub or central portion from which radiate outwardly a plurality of wires, sometimes in a woven configuration, the wires serving to filter clots from blood flowing through the vein. It is not uncommon for the central portion of the filter element, which is to be near the center of the lumen, to have the greatest efficiency, that is, to have wires that are closest together to thus produce smaller openings through which blood flows. The screen openings often become larger toward the walls of the vein. For good results, it is desired that the center of the filter remain near the center of the vessel, but often this is not easily controlled and the center of the filter may tilt to one side or the other of the lumen, thus exposing the less efficient areas of the filter to blood flow at the center of the lumen and reducing filtering efficiency.
Also, it is sometimes difficult to visualize with accuracy the effectiveness of a vena cava filter in filtering out blood clots. Commonly, an imaging or contrast medium such as Hipaque.RTM. (a product of Winthrop Pharmaceutical) or Conray.RTM. (a product of Mallinckrodt) may be injected upstream from the filter (that is, inferior to the filter in the vena cava) in the course of obtaining a vena cavogram, but the contrast liquid often becomes diluted in the blood stream as it reaches the filter, preventing precise visualization of blood clots in the filter. Moreover, to obtain a good vena cavogram, a large volume, e.g., 30 cc, of a contrast medium must be rapidly injected, and this procedure often is done with the aid of a diagnostic catheter.
Although, as thus described, a variety of vena cava filters have been suggested for use, the need remains for a filter that can be temporarily installed in the vena cava for later removal, and for a filter enabling blood clots captured by it to be readily visualized by the introduction of a contrast medium. It would be particularly desirable to provide a filter assembly capable of deploying an easily removable filter element, but that may also deploy an anchoring element proximal to (that is, downstream from) the filter element, as warranted by a patient's condition. It would also be desirable to provide a filter, the center or apex of which is positioned and maintained at or near the center of the lumen, so as to improve clot filtering efficiency. Moreover, it would be desirable to provide a filter assembly enabling a contrast medium or other fluid to be injected upstream from the filter elements.